1. Technical Field
The present invention relates in general to a compact, reliable, universal power distribution system and in particular to a power distribution system which incorporates surface mount technology in the manufacturing process. Still more particularly, the present invention relates to a controlled low impedance transmission line providing a high current capacity between a power supply module or a power supply circuit board and a digital signal processing module or a logic circuit board.
2. Description of the Related Art
Computer technology development has continually produced smaller and more compact data processing systems. State-of-the-art circuits now utilize low voltages to power digital logic circuits. A low voltage standard for digital logic which is gaining popularity is the 1.5 volt standard. However, power consumption has not declined with lower voltage standards. Unchanging power requirements in conjunction with low voltage standards mandates very high electrical currents.
For example, the power consumption of 10 amps at 12 volts is equivalent to the power consumption of 80 amps at 1.5v. Increased amperage requires the implementation of low impedance paths to distribute power because the resistive losses inherent in smaller conductors are prohibitive. These losses can produce unwanted heating which promotes component failure in computer systems.
Nearly all power supplies in today's computers are "switch-mode" power supplies. Switch-mode power supplies rapidly switch to chop the input voltage thus transferring a percentage of the input voltage to an output port. The output port is then filtered to average the output voltage, thereby stepping the input voltage down and producing a DC output. A switch-mode power supply regulates the stepped down voltage at its output port utilizing feedback loops. Losses due to the reactive characteristics of printed circuit traces and connectors can cause system malfunctions.
Further, the performance of switch-mode power supplies operating at high frequencies is degraded when the power transmission lines interconnecting a load such as a digital logic circuit becomes inductive. Inductive power transmission lines are particularly undesirable when high currents are present. Power transmission line inductance values which exceed one nanohenry can cause inefficiencies in power distribution performance.
Often, two component interconnect modules such as printed circuit boards (PCBs) or multi-chip modules require interconnection in close proximity. For example, in high density computer systems it is desirable to mount a power supply circuit board and a digital signal processing circuit board parallel to each other in close proximity.
"Pins" may be utilized between the power supply circuit board and the circuit board containing the data processing hardware to provide power to the data processing circuit board. A pin interconnect can create an inductance of 10 to 20 nanohenries and a capacitance of one picofarad on the interconnect line between the circuit boards. Additionally, relatively high resistances can be encountered. It would be desirable to reduce the inductance between circuit boards to less than a nanohenry.
Pin interconnects also provide minimal capacitance. For high currents, capacitance is desirable to reduce the ripple voltage on the output of the switch-mode power supply.
In radio frequency designs, transmission lines for radio signals are often implemented utilizing a "microstrip" configuration. A microstrip is a conductor separate from a ground plane utilizing an insulator made from a dielectric material. Microstrip designs allow the designer to accurately control the impedance of the transmission line. However, due to low current, radio frequency designs utilize feed through capacitors and do not utilize a microstrip configuration when interconnecting two circuit boards.
Generally, the interconnection of power distribution systems to digital logic circuits has not received the same amount of attention which other components of computer systems have. Power systems are necessary and an integral part of computer systems. Many power distribution systems utilize cables to interconnect power supplies to digital circuit boards.
Current computer construction techniques for power supplies require the assembly of cables and connectors. Hand soldering and/or hand crimping is required in almost every cable assembly which interfaces power sources to circuit boards, hard drives and other peripheral devices. Cabling requirements are costly and assembly is normally done by outside contractors who specialize in wire harnesses. Utilizing outside contractors and vendors requires purchasing effort, quality control, inspection and additional part numbers.
Pins and specifically square pins often require manual labor to insert the pins into a circuit board. Further, square pins generally require manual labor to hand solder the square pins to the printed circuit board. This problem arises because bulk soldering processes have a saw which cuts off excess component lead length and thus square pins would be cut off if soldered in a bulk soldering process. Both manual insertion and hand soldering are very labor intensive.
Hand soldering square pins also can create reliability problems. Specifically, quality control procedures to ensure quality in manual operations such as hand soldering, is an entire process in itself. It would be preferred to provide an automated soldering process to achieve power supply to digital circuitry power system interconnection.
Finally, current power interconnect designs require human labor to individually, electrically and mechanically connect power supplies to a system. All of the foregoing tasks add to the assembly costs of computer systems, and in particular add to the cost of final assembly of a computer system.
Circuit board manufacturing considerations such as tolerances related to the density of "through hole" and "trace" fabrication, prohibit high trace density. Hence, lengthy power busses are required on CIMs or in cable runs. Problems arise when power distribution lines having an uncontrolled impedance are utilized where low voltage digital technology is supplied by a switch-mode power supply. A small voltage drop in a low voltage power distribution system equates into a large percentage of voltage loss.
To reduce the overall size of electronics and distribution loss, designers have attempted to place digital logic circuits in close proximity to switching power supplies. However, high inductive and resistive impedance between switching power supplies and signal processing circuit boards has been a significant design problem.
Circuit boards or CIMs containing power distribution circuits can radiate electronic noise which originates from switch-mode power supplies. Most switch-mode power supplies operate at a determined frequency. When the length of the interconnecting transmission line reaches a fraction of the wavelength predetermined by the switching frequency, the power transmission line can act as an antenna. Radiating noise can interfere with low voltage digital logic circuits, this is particularly true when inductive or capacitive coupling occurs. Power transmission lines are present in every chip and circuit board and virtually everywhere in a computer system.
A power interconnect system which interconnects two circuit boards or CIMs and can accommodate high currents at low impedance would be both desirable and advantageous. Hence, there is a need for a low cost, high density and reliable power interconnect system having a controllable impedance. The present invention is directed at reducing the cost and size of power interconnect systems while solving reliability problems and accommodating high currents with a low impedance.